Waterproof cables are today in wide usage in both the communication and power transmission fields. One such cable used in the telecommunications industry has a core formed by a bundle of insulated wires which are encased within a tubular, metallic shield that enhances the structural integrity of the cable. The tubular shield is itself encased within an extruded polymer jacket that provides a waterproof covering.
To insure the structural integrity of the cable the shield should completely surround the core so as not to leave a longitudinal gap therebetween. In order to accomplish this the edges of the sheet metal are mutually overlapped as they are wrapped about the core into a generally hollow, cylindrical configuration. The exterior surface of the shield is thereby formed with a longitudinal seam providing by the outer, overlapping shield edge.
Once the core has been so shielded it is passed through an extruder which extrudes a cylindrical layer of a plastic such as polyethylene at an elevated temperature about the shielded core. This outer layer is then cooled whereupon the plastic solidifies into a cylindrical, waterproof jacket. When this occurs the inside surface of the jacket tends to assume the generally cylindrical configuration of that of the exterior surface of the metallic shield. Over the longitudinal seam of the shield however the jacket assumes a slight, but nevertheless distinct, steplike configuration over the exposed shield edge. In this manner a longitudinal discontinuity is formed along the inner surface of the extruded jacket.
Once the shield cable has been so manufactured it may become subjected to quite rough handling in the field. For example, where the cable is entrenched it is conventionally done so with a specialized cable laying plow. During this operation the cable is drawn off of a reel supported above ground and then directed in a generally vertical direction downwardly to the plow. At the plow the cable is rather abruptly redirected in a generally horizontal direction. This redirectioning of the cable causes it briefly to assume approximately a right angle turn or bend as shown in the side view of FIG. 4. This bending action causes the cable, which normally is of cylindrical configuration, momentarily to assume an eliptical cross-sectional configuration along a plane P as illustrated. The jacket portion of the cable at this point is thus subjected to substantial tensil forces along this plane. These forces can cause the jacket to form a split or rupture R as shown in FIG. 5. If the rotary orientation of the cable at this point happens to be such as to locate the previously described longitudinal discontinuity along the inside surface of the jacket at the convex, lower surface 10 of the cable, the probability of such a rupture occuring is quite substantial. It therefore is desirable to prevent any such discontinuity from being formed along the inside surface of the jacket over the shield seam during cable manufacture.
In an effort to overcome the just described problem a strip of laminated tape made of aluminum foil, Kraft paper, and Mylar film has heretofore been used in waterproof cable manufacture as an overlay to cover the shield seam. This strip of tape has inhibited the formation of a step along the inside surface of the jacket immediately above the overlapped, exposed edge of the shield seam. However, the side edges of the tape itself, located to each side of the seam, have themselves caused steps to be formed along the inner surface of the jacket. Indeed, where only one step was before present, use of the seam tape has led to the formation of two steps although of lesser height. In power cables, which are subjected to rapid changes in temperature and size over their lives resulting from cable usage, the edges of such seam covering tape have been tapered so as to provide a streamlined or feathered contour. This tape however is quite costly since it cannot be manufactured in master roll form but instead must be extruded in individual tape form.
Accordingly, it is a general object of the present invention to provide an improved cable of the type having a jacket extruded over a metallic shield formed with a longitudinal seam.
Another general object of the invention is to provide a method of manufacturing a shielded cable of the type just described.
More specifically, it is an object of the invention to provide a cable having an extruded jacket overlaying a metallic shield having a longitudinal seam with the inside surface of the jacket having a smooth contour over the shield seam.
Another object of the invention is to provide a method of manufacturing a cable having a metallic shield formed with a longitudinal seam about a cable core overlayed with a plastic jacket having a smoothly contoured inside surface overlaying the shield seam.
Still another object of the invention is to provide a cable and a method of making the same of the type described which is of relatively simple and economic construction and which is readily compatible with existing cable structures and manufacturing techniques.